Huzzah! I have a (VERY) basic level editor put together using WinForms and the XNA+WinForms sample code. It supports clicking to select and drag-drop operations within the editor pane on the right. Next up is adding a context menu or other method of managing what components are added to a given entity. I'm considering a PropertyGrid - it would give a relatively straightforward way to modify all of the attributes of the existing components and should be easily extensible.